Excavating Extinction Histories

Reference: Steadman, D.W., et al. (2019). The paleoecology and extinction of endemic tortoises in the Bahamian Arhipelago. The Holocene. I-8. https://journals.sagepub.com/doi/10.1177/0959683619887412

Learning from extinct tortoises

The Bahamas look much different today than they did before humans arrived. Aside from resorts and power lines, one big difference is that the modern Bahamas are lacking what were once their largest plant eating land animals – native tortoise species. Though none of these tortoises remain on the islands, fossils and cultural accounts have indicated their historic presence. To get a better idea of when and why they went extinct, and to learn about their role in the historical ecology of the islands, David Steadman and his colleagues investigated tortoise remains using modern techniques. By doing so, they have provided important information on the historic role of tortoises in island ecosystems as well as their demise.

Measuring the past

Radiocarbon dating (determining the age of materials based on the amount of a specific type of Carbon (carbon-14)) and stable isotope analyses (which can provide information on organisms’ diet based on elemental signatures, like the ratios of different forms of Nitrogen) were used by the authors to study the tortoises’ role in historical ecosystems. It may seem odd that aged bones could provide such valuable information, but collagen is often preserved for an extremely long time. Some of the bones they studied were even so well preserved that ancient DNA was successfully extracted and analyzed from them in another study (click here to read more about this)!

It is suspected that there were several species of tortoise restricted to specific parts of the Bahamas. These are thought to be new species to science, and are yet to be described. Because fossil records are unavoidably lacking (not every organism is fossilized, not all fossils are complete, and not all fossils have been found), there is much left unknown about the exact number and population sizes of species on the islands. In this study, dating information was gleaned from 16 tortoise bones (representing various species) taken from 6 different Bahamian islands. For diet analyses, 13 of the tortoise bones were measurable via stable isotope analysis.

Additionally, historical information was also gathered from archeological and cultural sources. The first humans are known to have arrived on the islands around 700AD, and the settlement of the rest of the archipelago had occurred by 1000AD. A group of people called the Lucayans lived in the Bahamas at that time (prior to European settlement).

New insights into history 

            Dating revealed that native tortoises lived on the islands within the last 3500 years, with some specimens dating a little after 1000AD (indicating they overlapped briefly with the Lucayan people). Overhunting by the Lucayan people appears to have contributed to the tortoises’ extinction. The youngest native tortoise specimen described by the authors is thought to have lived on Grand Turk island around 1200AD, which suggests the tortoises may have overlapped with humans on Grand Turk longer than on other Bahamian isles.

Regarding diet, most of the tortoise bones revealed isotopic values reflective of browsing land herbivores. Geographic patterns were detected across the island chain, and it appears that the diet of tortoises from the more southeastern islands fed on a greater proportion of marine derived foods or cacti than their more northern counterparts. The cacti based diet is given greater credence by the authors as the more southern islands are known to be drier, and cacti have isotopic signatures similar to those detected in the tortoise bones.

What this means for you and me

The ability to glean information about ecology and extinction history of ancient organisms from their bones is an exciting advancement in science. More personally, however, it allows us to get a clearer understanding of when and why organisms went extinct, and helps us understand how the ecosystem has changed in response to their extinction. Bahamian tortoises were most likely driven to extinction because humans overhunted them for meat, even before Europeans arrived. This speaks to the immense power and responsibility that we as humans have in our environment and to the organisms we interact with. Bahamian tortoises likely served an immense role in their ecosystems as seed dispersers, but they no longer can fill that role because of human actions. Going forward, I hope that the lessons we’ve learned from the Bahamian tortoises remind us to be responsible in our shopping, hunting, and harvesting practices. When purchasing animal (or plant) derived products, consider their source and the impacts that their harvest has on their populations and the ecosystems they inhabit.